As the new year approaches, it's a great time to think about your investing strategy. There's no guarantee that the market will deliver the great returns in 2021 that we've seen so far this year. However, there are reasons to be optimistic about the potential for another strong performance.

Although the S&P 500 index has risen by a double-digit percentage year to date, there are a lot of stocks that haven't done so well. It's important to pick the right stocks to add to your portfolio. But which stocks have tremendous potential? Here are three high-growth stocks that should make you richer in 2021 and beyond.

A person looking pensive with a thought bubble containing a picture of a money bag

Image source: Getty Images.

1. Guardant Health

Guardant Health (NASDAQ:GH) delivered relatively modest gains this year compared to 2019, when the healthcare stock more than doubled. Still, I suspect most investors are pretty happy with those "modest gains" of nearly 60% year to date.

The company is a leader in developing liquid biopsy products. Liquid biopsies are blood tests that analyze DNA fragments broken off from tumor cells. In August, Guardant Health became the first to win U.S. Food and Drug Administration (FDA) approval for a liquid biopsy used in comprehensive tumor mutation profiling across all types of solid cancers.

Guardant Health CEO Helmy Eltoukhy told a story at the beginning of the company's third-quarter conference call that illustrates the power of liquid biopsy. He mentioned a 44-year-old man diagnosed with late-stage lung cancer. The patient's oncologist initially put him on chemotherapy and immunotherapy Keytruda. Unfortunately, he didn't respond to this treatment. Before putting the patient on hospice, the oncologist ordered Guardant Health's Guardant360 test. The liquid biopsy identified a specific genetic mutation that gave the oncologist the needed information to put the patient on combo therapy from Novartis that was just what the patient needed.

The market potential for Guardant360 stands at $6 billion in the U.S. alone. However, Guardant Health is developing other products for early cancer detection and cancer recurrence monitoring that target an even greater addressable market of more than $45 billion.

I'm convinced that liquid biopsy will be a game changer in cancer treatment. And I'm optimistic that Guardant Health will be an even bigger winner for investors in the future than it's been so far.

2. MongoDB

MongoDB (NASDAQ:MDB) is sizzling-hot right now. Its shares have skyrocketed more than 150% year to date after jumping 57% in 2019. I think the heat will still be on next year.

The COVID-19 pandemic has accelerated companies' migration of their apps and data to the cloud. MongoDB's Atlas cloud database management system is a top choice for these companies. It's supported by all of the major cloud hosting companies. Atlas is the first (and so far only) cloud database that enables customers to run an application across multiple cloud providers simultaneously.

Atlas is the primary growth driver for MongoDB. Of the company's more than 22,600 customers as of Oct. 31, 2020, over 21,100 use Atlas. That figure is up 60% from the prior-year period.

The shift to the cloud isn't just a short-term phenomenon, though. MongoDB is likely to deliver strong growth for years to come. Speculation that the company could be acquired by Microsoft in 2021 isn't surprising. I wouldn't bank on such a deal. The good news, though, is that with MongoDB's great prospects, investors won't need an acquisition to receive a fantastic return from this database stock.

3. Trupanion

If you like the returns for Guardant Health and MongoDB so far this year, you'll probably love Trupanion (NASDAQ:TRUP). The pet medical insurance provider's share price has nearly tripled in 2020. Look for more good news in 2021.

I'm especially pumped about Trupanion's relationship with huge supplemental insurer Aflac. The two companies have teamed up to offer Trupanion's pet insurance alongside Aflac's insurance products in workplaces across the U.S. They're also exploring similar opportunities in Japan, where Aflac has a strong presence. Aflac likes Trupanion's prospects so much that it invested $200 million in the company. 

There's a good reason why Aflac chose to partner with and invest in Trupanion. The company's revenue continues to grow at a rapid pace -- 31% year over year in the third quarter of 2020. Trupanion's rising subscription volume and an average monthly retention rate of 98.7% is a formula for tremendous success. 

Despite its strong growth so far, Trupanion has barely scratched the surface of its opportunity. The market penetration for pet insurance in the U.S. is only around 1%. It's not much higher in Canada. In the U.K., though, around 25% of cats and dogs are covered by medical insurance.

I think Trupanion will steadily increase its market penetration in North America, especially with its Aflac partnership. This stock should make investors richer not just next year, but over the long run, too.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.